[SOLVED] PHP multiple clients accessing same file

PHP multiple clients accessing same file

I started playing with PHP, and saw this code snippet that is supposed to act a hit counter for a webpage:

/* counter */

//opens countlog.txt to read the number of hits
$datei = fopen("/countlog.txt","r");
$count = fgets($datei,1000);
fclose($datei);
$count=$count + 1 ;
echo "$count" ;
echo " hits" ;
echo "\n" ;

// opens countlog.txt to change new hit number
$datei = fopen("/countlog.txt","w");
fwrite($datei, $count);
fclose($datei);

Based on what I read, multiple requests can run simultaneously on the server. So there is chance that they access this file countlog.txt at the same time (correct?). If so, this code doesn't work for a busy website (correct?). How would one change this code to make it work for a busy website? Can you use locks in PHP that is shared between multiple request?

PS: The question is NOT about counters. Please avoid using SQL in the answer if possible.

Solution

You could use flock() to get an exclusive lock on a file, see http://php.net/manual/en/function.flock.php

$fp = fopen("/tmp/lock.txt", "r+");

if (flock($fp, LOCK_EX)) {
    $datei = fopen("/countlog.txt","r");
    $count = fgets($datei,1000);

    $count=$count + 1 ;
    echo "$count" ;
    echo " hits" ;
    echo "\n" ;

    ftruncate($fp, 0);
    fwrite($fp, $count);
    fflush($fp);
    flock($fp, LOCK_UN);
} else {
    echo "Could not lock file!";
}

fclose($fp);

Maybe you should build a loop that waits for a successfull lock using usleep() to avoid active waiting: http://php.net/manual/en/function.usleep.php